Fire extinguishing



United States Patent 3,475,332 FIRE EXTINGUISHING LeRoy J. Leeper, St. Paul Park, and Gunther H. Dierssen, White Bear Lake, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Aug. 12, 1966, Ser. No. 575,217 Int. Cl. A62d 1/00; A62c 3/06 U.S. Cl. 252-2 17 Claims ABSTRACT OF THE DISCLOSURE Metal fires are effectively extinguished by applying a composition comprising a mixture of (a) a minor amount of finely divided stable metallophile (e.g., carbon, silicon carbide or zirconium boride) and (b) a major amount of small inorganic bubbles having a lower density than the metal (e.g., glass bubbles).

This invention relates to the extinction of metal fires, particularly where the burning metal is in liquid form.

Conventional fire extinguishing materials are generally ineffective in putting out metal fires. Water, which extinguishes fire by lowering the temperature below that required to support combustion, is generally incapable of establishing contact with a burning metal because of the layer of insulating steam which develops. Water reacts violently with the alkali metals, and both Water and carbon tetrachloride react explosively with burning magnesium. Sodium carbonate, sodium bicarbonate, and carbon dioxide itself actually increase the rate of combustion for burning alkali metals. Foams similarly introduce oxygen and make alkali metal fires burn more vigorously.

For years the most common way to extinguish metal fires has been to throw on an excess of sand, vitrescible particles, or similar inactive materials, the metal continuing to burn while more or less confined until either it is dissipated, or, if it has a sufficiently high flame point, extinguished. Obviously, the effect is primarily containment, not extinction, and such materials are at best useful only where the burning metal is not in liquid form. Sand applied to burning alkali metals, which are both low-density and low-melting, sinks and spreads the fire over a wider area.

Prior to the present invention, the most elfective and widely used commercial agents for extinguishing metal fires have been of two general types-(1) sodium chloride particles, treated with metal soaps or polysiloxane resins to facilitate flow, and (2) a blend of powdered graphite and granite particles. These materials are effective to some degree on such burning solid metals as aluminum, but where the main body of the metal is liquid during the process of combustion, the presently available commercial extinguishing agents serve only to contain the fire, which is likely to break out uncontrollably over a period of several days after it is nominally extinguished. Salt functions by fusing into an oxygen-excluding crust, but the almost inevitable presence of Water frequently causes explosive reaction when salt is applied to burning lithium, sodium, potassium, or alloys of such metals. Crystallization of the cooling salt also opens gaps through which moisture or oxygen can penetrate to regnite the presumably extinguished fire. NaCl is particularly ineffective on potassium fires, and the use of KCl is precluded by its moisture-absorbing nature. Graphite, silica, and granite are all heavier than alkali metals and hence, even though the graphite may compete for oxygen to some extent, these blends are not very successful on alkali metal fires.

The most common alloys of sodium and potassium (NaK) are liquid at room temperature, a characteristic which makes NaK suitable for use as heat exchange fluid in atomic reactors or high temperature processes, as cooling liquid in power transformers and molding operations, as hydraulic fluid, and in many other industrial applications. The use of NaK alloys has been greatly limited, however, by the fact that there has previously been no material rated by Underwriters Laboratories for the extinction of sodium, potassium, or NaK fires.

The present invention provides a novel, simple, and remarkably effective extinguishing agent for NaK fires, which are considered the metal fires most difiicult to extinguish. The product of this invention can also be employed on other metal fires with at least equally satisfactory results.

The fire extinguishing agent of this invention comprises a free-flowing at least predominantly inorganic particulate composition of matter essentially comprising a mixture of (a) finely divided stable metallophile and (b) stable inorganic bubbles. Stable is used in its conventional chemical sense, i.e., not decomposing readily in the presence of the burning metal which is to be extinguished.

It is recognized that inorganic bubbles have previously been suggested for extinguishing fires, although we believe they have never been used on liquid metal fires. It is also recognized and that such metallophiles as graphite have previously been used (albeit rather ineffective) on alkali metal fires. We have found, however, that a particulate blend of a major amount of hollow glass spherules and a minor amount of metallophile has a synergistic effect in the extinguishing of alkali metal fires. The reason for this phenomenon is frankly not known.

The metallophile, which constitutes no more than a minor part of the composition (preferably less than 10% by volume), is an inorganic substance which is Wetted by the molten metal to be extinguished, but which is stable under the conditions of use, e.g., neither nitrides, which react violently with alkali metal, nor titanium, which is itself highly combustible, are considered suitable for use on alkali metal fires. Suitable metallophiles frequently comprise an element selected from Group IV of the Periodic Table, either alone or in combination with other elements, e.g., graphite, silicon carbide, zirconium diboride, etc. The particle size of the metallophile is preferably small enough, both to prevent rapid sinking in a burning liquid metal which is to be extinguished and to permit adhesion perhaps by static charges, to the bubbles which constitute another significant part of the composition. These particles should thus not exceed about 50 to microns effective diameter, and preferably should be of submicron size. The metallophile should also be dense and nonporous, so as not to supply adsorbed combustion aids (e.g., air or moisture) to the burning metal. Thus, when carbon is employed as a metallophile, it should be in the form of dense, fine particles or flake graphite, rather than in. the form of porous activated charcoal, or similar material.

The bubble portion of the composition of this invention is made up of substantially unicellular hollow spherules. Preferably these bubbles are made of ingredients which either constitute a glass per se or which, in combination with alkali and/or alkali oxides, are capable of forming a glass. Effective bubbles may be formed from soda-lime borosilicate glasses, ceramic spherulized clay particles as described in U.S. Patent 2,676,892, etc. The effective density of the bubbles should be less than that of the burning metal, and it is desirable that their particle size fall in the approximate range of 25 to 1000' microns, exceeding that of the metallophile.

The bubblecmetallophile ratio and extinguisherzmetal density ratio should be adjusted, and to some extent correlated with the viscosity of the molten metal, so that the fire extinguishing composition will remain at or near the surface of the burning metal, or sink only slowly. Generally speaking, the larger the hollow glass spherules, the less their tendency to carry adsorbed oxygen and moisture on their surfaces and thus cause flashing when initially applied to a fire, provided the wall thickness is at least on the order of 1 micron. On the other hand, large spheres with thin walls contain greater quantities of gas in their interior. Evacuating the spherules or incorporating a non-reactive gas such as argon or helium therein, enhances their effectiveness.

The simplest form of this invention involves a 2- component product, a hollow glass bubble and a finely divided metallophile. If desired, however, a composite metallophile, e.g., a blend. of silicon carbide and flake graphite, may be employed. Similarly, a blend of glass and/or ceramic spherules of different composition may be used. Generally speaking, metallophile-bubble compositions of the type just described are free flowing, but they may be treated, if desired, with a thin coating of a stearate or silicon to facilitate flow under varying types of application conditions. The amount of such organic material should, however, be held to a minimum, since alkali metals react with even such normally inert organic materials as the fluorocarbons.

The following illustrative embodiments of the invention will facilitate understanding thereof without imposing any limitations upon its scope. All parts and percentages are by weight unless otherwise noted.

In Examples 1-4 inclusive, 50 grams each of sodium and potassium metal were heated together in a fused quartz dish to form an alloy having a melting point of approximately 10 C. The alloy was then ignited with a. propane torch and allowed to burn for three minutes, the flame temperature approximating 950 C. (If a draft is supplied, temperature of the flame reaches about 1250- 1300 C. in three minutes.)

EXAMPLE 1 To the fire was applied 125 grams of a mixture containing 75% by weight 25-35 micron soda-lime boro- 'silicate glass bubbles and 25% by weight (1.1-0.5 micron silicon carbide particles. The glass bubbles, commercially availablefrom Minnesota Mining and Manufacturing Company under the trade designation 3M glass bubbles, No. 35, had an average individual bubble density of 0.316; density of individual silicon carbide particles was estimated at theoretical 3.2. From these density figures, the fire extinguishing mixture was calculated to contain approximately 97% glass bubbles and 3% silicon carbide by volume. Within 30 seconds after applying the mixture to the surface of the fire, the fire was contained, a clinker-like crust forming in 3 /2 minutes. The fire was completely extinguished after 5 minutes and could not be reignited, even when cc. of water was sprinkled on the crust.

EXAMPLE 2 Over a 40-second period 16 tablespoons (71 grams) was applied of a mixture containing 80% 20-80 micron glass bubbles and 20% 0.1-0.5 micron silicon carbide. Calculated bubblezSiC volume ratio was about 97.7:2.3. There was a smoky flare-up when the mixture was first added, but the fire quickly crusted over to form a hard clinker which remained intact and did not burn through.

When the experiment is repeated, substituting an equal weight of glass beads having the same composition and size as the bubbles, flare-up is minor, but the composition sinks and spreads the fire.

When the experiment was repeated using 72 grams of a fire extinguishing agent containing only glass bubbles, a crust was formed quickly, but the fire later broke through the crust and continued burning. Application of silicon carbide granules alone, even in an amount equal to twice the weight of NaK present, cools the fire slightly, but the molten metal crawls up the sides of the dish and spreads the fire.

When the experiment was repeated using 91 grams of a commercially available extinguisher for metal fires (essentially stearate-coated NaCl grains), some spitting occurred initially and the fire broke through the crust which was formed, continuing to burn.

EXAMPLE 3 Ninety-four grams of a mixture containing 100 parts 40-micron glass bubbles (density 0.40) and 6 parts 0.1- 0.5 micron SiC was applied. Calculated bubble:SiC volume ratio was about 99.2:0.8. The fire was controlled in 1-2 minutes; a hard crust, which did not burn through formed in 3 minutes.

EXAMPLE 4 Fifty grams of a mixture containing 75% 40-rnicron glass bubbles and 25% 10-200 micron dense carbon powder was applied. Calculated bubblezcarbon volume ratio was about 96:4. Some initial flashing was noted, but the fire was controlled after 30 seconds, a crust forming in 3-5 minutes. The container had cooled enough to be handled after 25 minutes.

In Examples 5-10, 25 grams of sodium and 25 grams of potassium were melted, ignited, and pre-burned for three minutes, after which the fire extinguishing composition was applied.

EXAMPLE 5 Forty-five grams of a mixture containing 67% glass bubbles as in Example 1, 13% 0.1-0.5 micron silicon carbide and 20% 10-100 micron flake graphite (com mercially available from the Dickson Company as No. 635), was applied. (The relative volume percentages of glass bubbles, silicon carbide, and graphite, were calculated to be respectively 93.8%, 1.8% and 4.4%.) The fire was contained after one minute, a semi-hard crust forming within five minutes. The fire was totally extinguished at the end of 20 minutes. Increasing the weight percentage of graphite to about 30% while holding the bubble:SiC ratio constant (bubble:SiC:C volume ratio of 91.5 1.5 :70), results in the formation of a somewhat softer crust with equally effective fire extinction. It is felt, however, that a hard crust is more easily cleaned up and hence compositions containing lower amounts of graphite are preferred.

EXAMPLE 6 Fifty grams of a mixture containing of the glass bubbles employed in Example 1 and 20% of 10-100 micron graphite flake was applied. Bubblezgraphite volume ratio was calculated to be about 96:4. The fire was contained within 1 /2 minutes and completely extinguished at the end of 30 minutes.

EXAMPLE 7 Forty-eight grams of a mixture containing 83% 0f the glass bubbles employed in Example 1 and 17% -325 mesh 44 micron) zirconium diboride was applied. BubblezZrB volume ratio was calculated to be about 99:1. The fire was contained within 30 seconds, a very hard crust forming within 5 minutes. Total extinction had occurred at the end of 30 minutes. The high density of the ZrB (about 6.1) suggests that compositions of this type would prove useful in extinguishing fires involving such dense metals as plutonium.

EXAMPLE 8 Forty-one grams of a mixture of gl-ass bubbles and silicon carbide of the type referred to in Example 1 (except that the mixture had been previously introduced into a chamber, maintained at 1 mm. pressure for 20 minutes, flushed with helium, and kept in a helium atmosphere for 20 hours) was applied. The fire was contained within 15 seconds, complete extinction occurrring somewhere between 7 /2 and 15 minutes. This example shows the desirability of excluding from the fire extinguishing composition any gaseous material capable of supporting or promoting combustion. Helium and other Group VIII gases are unreactive with alkali metals, whereas air, carbon dioxide, and even nitrogen, tend to supply oxygen or react and hence to restrict the eifectiveness of a fire extinguishing composition.

EXAMPLE 9 Twenty-eight grams of an 80:20 mixture (97.7:2.3 calculated volume ratio) of 100-250 micron glass bubbles (density 0.298) and 0.1-0.5 micron silicon carbide were applied. The fire was completely contained within 15 seconds, a hard crust forming within 4 /2 minutes; complete extinction occurred within 15 minutes.

EXAMPLE 10 Example 9 was repeated, using a composition having a bubblezsilicon carbide Weight ratio of 83:17 (98.1:1.9 calculated volume ratio). Results were equally effective.

EXAMPLE 11 One hundred grams of 100-mesh titanium powder was ignited in a porcelain dish, attaining a flame temperature of 1800 C. after 1 minute. The glass bubblezSiC mixture used in Example 9 was applied, containing the fire within 1 minute. A hard crust soon formed, and the fire was extinguished within minutes.

EXAMPLE l2 Ninety grams of zirconium powder was ignited in a porcelain dish, and the glass bubblezSiC mixture used in Example 9 applied. A white ash, but no crust, was formed; the fire was contained.

EXAMPLE 13 Ninety grams of 8-mesh aluminum powder was ignited in a porcelain dish and the glass bubblezSiC mixture of Example 9 applied. A hard crust was soon formed, extinguishing the fire.

EXAMPLE 14 Forty grams of lithium was ignited in a porcelain dish and allowed to burn for 2 minutes, attaining a flame temperature of 1,350" C. The glass bubblezSiC mixture used in Example 9 was applied, containing the fire immediately. A hard crust formed in 5 minutes, and the fire was considered extinguished after 15 minutes.

When this example is repeated using the commercial 86:14 graphite: granite blend as the extinguishing agent, the fire comes through the layer of powder until a great excess of agent is applied. Even so, when the layer is disturbed after 15 minutes, the fire reflashes violently.

When this example is repeated using the commercial stearate-treated sodium chloride granules, a violent reaction takes place immediately.

EXAMPLE 15 One lb. each of sodium and potassium were melted in a 10-inch diameter steel frying pan, ignited with a propane torch and allowed to burn for 6 minutes. To the fire was then applied 2 lbs. of the glass bubble:SiC mixture used in Example 9. Within 36 seconds the fire was contained, and a hard crust formed within 3 minutes. When nearly a half cup of water was sprinkled on the crust after minutes, hissing was noted, but no violent reaction took place. The container was warm to a gloved hand after 30 minutes, and extinction was considered complete.

Two 2-lb. fires prepared by melting, igniting and me burning 1 lb. each of sodium and potassium in the manner just described were treated with commercial metal fire extinguishers.

When 8 lbs. of stearate-treated sodium chloride grains was applied to one fire, a soft crust formed, but the fire broke through within 30 seconds; an additional 2 lbs. of this extinguisher was then applied, but the fire again broke through after 15 minutes.

When 8 lbs. of a powdered 86:14 graphitezgranite blend was applied to the other fire, a large cloud of smoke arose, and the burning metal began climbing the sides of the frying pan. Additional extinguisher was applied; a soft crust formed and the fire was apparently contained after 3 minutes, but broke through the crust after 10 minutes. More extinguisher was applied, but whenever the crust was disturbed, the fire broke out again.

EXAMPLE 16 One lb. of sodium was melted in a 10-inch steel frying pan, ignited and allowed to burn for 6 minutes. To the fire was then applied /2 lb. of a mixture of 40- micron glass bubbles and 20% 0.1-0.5 micron silicon carbide. The fire was largely contained within 35 seconds, and a very hard crust formed within 1 minute. The container felt warm to a gloved hand within 20 minutes, and the fire was considered extinguished.

To a similar l-lb. sodium fire' was applied 4 lbs. of a commercially available composition recommended for extinguishing metal fires and consisting essentially of stearatetreated sodium chloride grains. The fire was contained within 30 seconds and a soft crust formed within 5 minutes; when crust was disturbed the fire was found to be still burning beneath. After the fire was allowed to cool ten minutes more, a few sprinkles of water were applied; a violent reaction was noted.

EXAMPLE 17 Example 16 was repeated, using potassium instead of sodium. When 350 grams of the glass bubblezSiC mixture used in Example 16 was applied, the fire was contained, and a hard crust formed within 1 minute. The container felt quite hot to a gloved hand after 15 minutes and warm after 30 minutes.

To a similar l-lb. potassium fire was applied 4 lbs. of stearate-treated sodium chloride grains as in Example 12. A soft crust was formed, but the fire soon broke through. Two more lbs. of the treated salt was then applied, but the fire again broke through after 5 minutes and continued burning vigorously for over 35 minutes, at which time the experiment was abandoned.

EXAMPLE 18 One hundred fifty grams of magnesium turnings were placed in a pile on a steel plate and ignited with a propane torch, a white-hot fire resulting in 10 seconds. About 75 grams of the glass :bubblezSiC mixture used in Example 9 was then applied, the fire being contained almost immediately and extinguished in less than 10 minutes.

What we claim is:

1. A free-flowing at least predominantly inorganic composition of matter for extinguishing a metal fire, said composition consisting essentially of a mixture of (a) a minor amount of finely divided stable metallophile having an average particle size of no more than about microns and (b) a major amount of small stable glass or ceramic bubbles which have a specific gravity less than that of said metal and which do not react violently with said metal.

2. The composition of claim 1 wherein the bubbles are glass bubbles.

3. The composition of claim 2 wherein the glass bubbles contain an inert gas.

4. The composition of claim 1 wherein the metallophile comprises an element selected from Group IV of Mendeleevs Periodic Table of the Elements.

5. The composition of claim 4 wherein the metallophile 1s an inorganic carbide or boride.

6. The composition of claim 4 wherein the metallophile 1s zirconium diboride.

7. The composition of claim 5 wherein the metallophile is silicon carbide having a particle size less than 50 mlcrons.

8. The composition of claim 4 wherein the metallophile is finely divided dense carbon.

9. The composition of claim 8 wherein the carbon is the form of graphite flakes.

10. A free-flowing at least predominantly inorganic composition of matter for extinguishing a metal fire, especially a molten alkali metal fire, consisting essentially of a mixture of 1-10 parts by volume of stable, particulate metallophile having an average particle size of no more than about 100 microns and correspondingly from 99-90 parts by volume of small glass-bubbles having an average particle size greater than that of said metallophile and a specific gravity less than that of the burning metal to be extinguished.

11. The composition of claim 10 wherein the metallophile consists essentially of silicon carbide having an average particle size of less than 50 microns and the average diameter of the glass bubbles is more than 25 microns.

12. 'The composition of claim 11 wherein the silicon carbide is predominantly of submicron particle size.

13. A free-flowing inorganic particulate composition of matter for extinguishing a metal fire consisting essentially of a mixture of silicon carbide, glass bubbles, and dense carbon, the glass bubbles constituting the major portion of said composition on a volume basis.

14. A free-flowing predominantly inorganic composition of matter having particular utility in extinguishing molten alkali metal fires, consisting essentially of, -99 parts by volume of glass bubbles having a specific gravity less than that of said metal and an average particle size in the approximate range of 25-1000 microns, 1-5 parts by volume of submicron silicon carbide and O to 5 parts by volume of dense carbon particles having an average particle size of no more than about microns.

15. A method for quickly and effectively extinguishing an alkali metal fire comprising applying thereto the composition of claim 1.

16. A method for quickly and eifectively extinguishing an alkali metal fire comprising applying thereto the composition of claim 10.

17. A method for quickly and effectively extinguishing an alkali metal fire comprising applying thereto the composition of claim 13.

References Cited UNITED STATES PATENTS 2,294,532 9/1942 Fahey et al 2522 2,368,209 l/l945 Fahey et a1. 252-2 2,937,990 5/1960 Warnock 2522 3,090,749 5/1963 Warnock 2522 MAYER WEIN-BLATI, Primary Examiner 

